This invention relates to a charge transfer imaging device.
Solid-state imaging devices using charge transfer devices (CTD) such as charge coupled devices (CCD) are recently extensively used because they have many advantages such as ease of size reduction.
The CTD has photosensitive sites such as PN diodes formed in superficial portions on a p-type semiconductor substrate for generating and storing signal charge according to incident light. The signal charge stored in the photosensitive sites is transferred to a charge transfer shift register by opening a shift electrode or transfer gate after a predetermined period of time. Then, after closing the shift electrode, the transferred signal charge can be read out as an electric signal from an output terminal by impressing a clock pulse on transfer electrodes.
This CTD, however, has a drawback that where high intensity light is irradiated and the period of reading from the shift register is long or where the incident light to be photoelectrically converted is of a long wavelength range near infrared, electrons generated in the semiconductor substrate other than the photosensitive sites are liable to flow into the shift register to deteriorate the image signal.
To overcome this drawback, there has been proposed an overflow drain, i.e., a reverse biased n-region buried under photosites arranged in p-layer, as disclosed in H. Goto et al, "CCD Linear Image Sensor with Buried Over-flow Drain Structure," Electronics Letter, 26th November 1981, vol. 17, No. 24, pp 904-905. The overflow drain here serves to discharge electrons generated outside the photosites. Thus, undesired carriers can be prevented from entering the shift register, and thus it is possible to alleviate the deterioration of image. Further, it is possible to prevent a blooming effect that may otherwise result when the photosites are saturated.
In this structure of CTD, however, it is liable that the carriers which are to be stored in the photosites partly flow into the overflow drain. Particularly, where the incident light to be detected is of a long wavelength range near infrared, the charge tends to be discharged to the overflow drain, thus lowering the sensitivity.